Transformation of T lymphocytes requires the participation of accessor cells. Starting from a rat lymph node cell preparation we have purified accessory cell activity more than 1000-fold, with an overall yield of 50-70%, and find that the dendritic cell (which makes up greater than 90% of the final preparation) accounts entirely for this activity. These cells are extremely potent; one dendritic cell per 200 periodate-treated lymphocytes produces maximum responses. Rat lymph node dendritic cells are also extremely effective stimulator cells in a mixed lymphocyte reaction. In contrast, macrophages purified free of dendritic cells from the cells from the same source are inactive in these assays. Because rat dendritic cells and macrophages have widely different properties that lead to their purification free of contamination by the other cell type and because the lack of activity of macrophages is unexpected, we want to expand our studies on these two cells to rat spleen, thymus, and peritoneal exudate cells. We plan to examine their ability to act as accessory cells, and to stimulate in a mixed lymphocyte reaction and in indirect stimulation (in which the stimulator cell is oxidized by periodate, galactose oxidase (GO), or neuraminidase plus galactose oxidase (NGO)). We also wish to determine the biochemical levels of a number of subcellular constitutents for dendritic cells and macrophages, preparatory to obtaining a plasma membrane-enriched fraction for studies on glycosylated cell surface molecules. Oxidation of these molecules profoundly affects the ability of dendritic cells to function more effectively. These molecules will be identified by a sensitive labeling method that we have developed which involves the reaction of 35D-cysteine methyl ester with aldehydes formed after oxidation with periodate, GO, or NGO. Ia antigens will also be measured qualitatively and quantitatively on macrophages and dendritic cells, and the results of this study and those of the labeling experiments correlated, if possible, with the immunological functions of these two cells.